Troy LaFerrara, 42, was stabbed and strangled to death by 19-year-old Miranda Barbour and her husband, Elytte Barbour, 22, in Pennsylvania

A satanic teen killer and her husband have avoided the death penalty after they pleaded guilty to fatally stabbing a married man whom she lured to his death using the Internet.

Miranda Barbour, 19, and Elytte Barbour, 22, had previously pleaded not guilty to first-degree murder charges that carried a possible death penalty upon conviction.

Following her arrest the 19-year-old boasted from her prison cell on Valentine's Day she had killed at least 22 other people over six years in a cross-country murder spree motivated by her satanic cult beliefs.

She said she dumped body parts in Big Lake, Alaska, and also in Mexico Beach, Florida where she worked as a 15-year-old lap dancer but police have failed to find any victims.

Sydney Morning Herald

Married: Police said the couple murdered LaFerrara on their three-week wedding anniversary and Elytte's 22nd birthday

On Monday the couple changed their pleas to guilty of second-degree murder at what was to have been a routine status conference in Northumberland County Court in Sunbury.

The couple married in North Carolina and moved to Pennsylvania about three weeks before they met up with the victim, Troy LaFerrara (corr), when he accepted her offer of sex in exchange for money.

Police say Barbour stabbed the engineer about 20 times in her parked car as her husband held a cord tight against his neck from the back seat and then dumped his body in an alley.

They had met after she advertised for a “companionship” on the Internet in exchange for money.

Investigators had been left shocked by the brutality of the murder which they have called a “thrill kill” to mark the couple's three-week wedding anniversary.